The Collaborative International Dictionary of English v.0.48:

Rear \Rear\, a.
   Being behind, or in the hindmost part; hindmost; as, the rear
   rank of a company.
   [1913 Webster]

   Rear admiral, an officer in the navy, next in rank below a
      vice admiral and above a commodore. See Admiral.

   Rear front (Mil.), the rear rank of a body of troops when
      faced about and standing in that position.

   Rear guard (Mil.), the division of an army that marches in
      the rear of the main body to protect it; -- used also
      figuratively.

   Rear line (Mil.), the line in the rear of an army.

   Rear rank (Mil.), the rank or line of a body of troops
      which is in the rear, or last in order.

   Rear sight (Firearms), the sight nearest the breech.

   To bring up the rear, to come last or behind.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rear admiral
    n 1: an admiral junior to a vice admiral